Understanding the role of precipitation intermittency on changes in extreme flooding

Extreme precipitation is projected to increase in many parts of the world, leading to concern over flooding and associated hazards. Despite the intuitive causal link between extreme precipitation and extreme flooding, observations and models suggest that other factors, in particular, low antecedent moisture may obscure or offset this relationship. Low antecedent soil moisture can arise through evaporation—driven by atmospheric warming that is projected to continue—but moisture is also reduced through an increase in the length of the time between storms, i.e., ‘storm intermittency’, which has been less studied and which can be derived directly from precipitation data. In this presentation, we focus on the modulating role of storm intermittency on the relationship between extreme precipitation and both flood potential, i.e., both flood peaks and volumes. We create an observation-based historical baseline of storm intermittency, 1950-2015, from a set of case-study basins to understand the relationship across a range of hydrometeorological settings. Next, the storm intermittency from a set of 16 CMIP6 climate models is evaluated relative to the baseline, and the evaluation is used to project future intermittency and likely outcomes on flood potential for the period 2016-2100. This projected flood potential is compared with hydrologic simulations from downscaled land surface models forced by the same CMIP6 models and differences between the projected and modeled outcomes are assessed in the context of simulated soil moisture and other hydrologic factors. Overall, we seek to understand the utility of storm intermittency as a predictor of flood potential and to understand the impacts of projected intermittency on future flood hazards.